EPR evidence for one-electron oxidation of phenols by a dimeric manganese(IV/IV) triazacyclononane complex in the presence and absence of hydrogen peroxide

Abstract

The reaction of [L₂Mn₂^IV(µ-O)₃](PF₆)₂, L = 1,4,7-trimethyl-1,4,7-triazacyclononane (1) with a range of phenols (2–5) in aqueous solution at pH 10.5 has been investigated. At least for electron-rich substrates, the reaction proceeds via a rapid overall one-electron process from the phenolate ion to the Mn^IV/Mn^IV species (1) to give, initially, a Mn^III/Mn^IV species (detected via its characteristic 16-line EPR spectrum in frozen solution at 77 K) and the corresponding phenoxyl radical, detected directly for Trolox (3) by EPR spectroscopy in aqueous solution. The dimeric Mn^III/Mn^IV species is ultimately converted to monomeric Mn^II. In the presence of H₂O₂, reoxidation of the manganese species is accompanied by an increase in the rate of formation of the phenoxyl radicals. In similar reactions, 4-methoxyphenol (4) and 2,6-dimethoxyphenol (5) are converted into polyphenols (with the formation of phenoxyl radicals trapped in a polymer matrix). Kinetic EPR and UV–VIS studies provide additional evidence of the reaction mechanism in the presence and absence of H₂O₂. The mechanism of phenolate oxidation by 1 in the presence and absence of H₂O₂ is discussed.